Yes, let the plaque dry on both sides, flip it over about every 20 minutes till it is completely dry. If you put the edible image on while the gumpaste is still slightly soft I think the adhesive on the image will prevent it from drying and the plaque may warp or sag.
Actually when I do mine, I trim my image to outline the shape, attach it by brushing water on to a piece of freshly rolled out gumpaste and then trim the gumpaste around the image. Then I set it on a flat surface dusted with cornstarch and let it dry. Works perfectly.
Here is little tutorial that boween did awhile ago: http://forum.cakecentral.com/cake-decorating-ftopict-13474.html
She uses piping gel to attach, but water works fine for me.
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